This invention pertains to pumps and, more particularly, to jet pumps used for circulating water in a jetted tub.
Jetted tubs are tubs that are outfitted with a pump for re-circulating the water in the tub. Typically, a pump takes water from the tub and re-circulates the water back into the tub through one or more jets, which are mounted on or in the tub. The jets are typically directed toward a portion of a tub user's body for recreational and/or therapeutic purposes. The user can aim the jets or position himself or herself in front of the jets. In some jetted tubs, air can be introduced into the jets along with the water to increase the therapeutic effectiveness of the jets.
Jetted tubs, are typically household sized bathtubs, as distinguished from larger sized hot tubs configured for simultaneous use by multiple users. For both types of tubs, bacterial growth is an important health problem. Typically, hot tub water is treated with chlorine or bromine to reduce the capacity of the water to support bacterial growth. However, a jetted tub is typically filled with minimally treated household water. While larger sized hot tubs are usually kept full of heated water, the smaller jetted tubs are usually drained after every use. It is therefore important that jetted tubs, including the pump and all of its associated plumbing, drain completely so as to leave no remaining water, which might support bacterial growth.
Centrifugal pumps are commonly used for re-circulating the water in a jetted tub because they are generally efficient, reliable, and low cost. A typical centrifugal pump comprises an electric motor that turns a generally disk-shaped impeller having an inlet opening near its center. When the impeller is rotated, water inside the impeller is also caused to rotate. The resulting centrifugal force of the water causes it to move from the center of the impeller near the inlet opening outwardly along vanes toward the perimeter of the impeller. Water exiting the perimeter of the impeller is collected by a generally annular volute, which channels the water to a pump outlet. Thus, the centrifugal force created by the impeller causes a pressure differential between the pump inlet and pump outlet, which causes water to flow through the pump. The flow of the pump is a function of both the speed of the motor and the size of the impeller. The faster the motor spins the impeller and the larger the diameter of the impeller, the greater the difference in pressure between the pump inlet and pump outlet.
FIG. 8 shows a typical prior art pump P. In a typical installation, the pump inlet I is connected to, and draws water from, the tub outlet or drain. The pump outlet (not shown) is connected to the one or more jets, which are typically mounted to the side walls of the tub. In a typical mode of operation, the user fills the tub with water and then turns on the pump. When finished, the pump is turned off and the tub is then drained. When a jetted tub is drained, the water in the various plumbing parts drains down into the pump outlet, backwards through the pump and out the pump inlet I to the tub drain. The lower half of a typical centrifugal pump casing is below the level of the centrally located pump inlet I and, therefore, water in the lower half of the pump casing is unable to drain out through the pump inlet I. To avoid leaving water in the lower half of the pump casing, which might support bacterial growth, typical pump casings include an extended, downwardly sloping inlet pipe E with a built-in offset, so that a bottom portion of the inlet pipe E is level with the bottom of the pump casing. A small bypass tube T connects the lower portion of the pump casing to the bottom portion of the inlet pipe E to allow water that would otherwise be trapped in the lower half of the pump casing to drain when the tub is drained.
Again, the flow of the pump is a function of both the speed of the motor and the size of the impeller. Typical prior art jet pumps are driven by an electric motor that operates at or above 3,000 rpm. When the pump is driven by an electric motor that operates above 3,000 rpm, the required flow can be achieved with a relatively small impeller. In such high-speed pumps, the volute and pump casing are relatively small and, accordingly, the necessary inlet offset is relatively small. A relatively small inlet offset can be easily molded into the front of the pump casing. In the molding process, the inlet pipe is typically "cored" by two pins, one from each side of the mold. The two pins are offset from one another by the necessary amount, but still meet in the middle with enough contact to create the necessary passage through the inlet pipe.
Noise is a common problem with jetted tubs that use such high-speed pumps. In general, fractional horsepower motors which operate at less than about 2,000 rpm are, for the same horsepower, less expensive and quieter in operation. The drawback to using a lower speed motor is that a larger impeller is required to maintain the same pump performance. A larger impeller requires a larger pump casing. The offset in the inlet pipe that is required to drain the pump casing also becomes larger and impossible to mold by the "coring" process described above. This is because the two pins would be offset from one another by too much to meet in the middle with enough contact to create the necessary passage through the inlet pipe. Consequently, the inlet pipe must be constructed in two pieces that need to be first assembled to one another, and then assembled to the rest of the pump casing.
Another problem is uneven water velocities. While the large inlet offset and bypass tube solve the drainage problem, the large offset creates an uneven distribution of water velocities. When presented to the inlet of the impeller, these uneven velocities degrade pump performance significantly.
Thus, there is a need for a pump casing that can be used with a lower-speed motor, with an inlet offset: that can be easily molded into the pump casing; that is sufficient to permit a drain "leak" to be placed between the bottom of the pump casing and the bottom of the inlet pipe; and that does not compromise pump performance by creating uneven flow velocities.